With the growth of the wireless communications industry, wireless communications protocols become more sophisticated and demanding in their requirements for complex modulation schemes and narrow channel bandwidths. One such communications protocol is wideband code division multiple access (WBCDMA), which requires modulation signals with square-root raised cosine (SRRC) pulse shapes. To provide complex modulation signals, digital modulation methods may be necessary.
Wireless RF transmitters generally need to provide stable output power with minimal distortion under varying antenna loading and environmental conditions. A quadrature-based transmitter architecture is one way to provide stable output power under varying conditions. A quadrature-based architecture uses an in-phase signal path and a quadrature-phase signal path, which are phase shifted and combined to form an RF output signal.
Quadrature RF transmitters with low distortion, digital quadrature modulation may be optimal for some communications systems. Some quadrature RF transmitters utilize modulation data that is provided at one data rate; however, system clock constraints may require modulation data at a different clock rate for digital filtering and other processing of the modulation data. For example, in a multi-mode universal mobile telecommunications system (UMTS) transmitter, enhanced data rates for global system for mobile communications (GSM) evolution (EDGE) transmissions use a symbol rate of 270.833 khz, which is a sub-multiple of 26 Mhz; however, WBCDMA transmitters use a chip rate of 3.84 Mhz, which is not a sub-multiple of 26 Mhz; therefore, some form of rate conversion is required. A chip is a bit that is sent in a spread spectrum communications algorithm. Several chips may be required for each bit of information to be sent. Chip rate refers to the number of chips per second that are sent in a spread spectrum communications algorithm. To rate convert modulation data, some kind of interpolation algorithm must be used to take input modulation samples at one data rate and calculate the proper values for output modulation samples to be provided at a different data rate. During the rate conversion and interpolation process, distortion of modulation information must be minimized.
Some communications systems require adjustments to the timing of reply transmissions relative to the timing of received information. Such adjustments are part of a feature called timing control and are referred to as timing advance/delay. For example, WBCDMA systems may require timing advance/delay in ¼ chip increments.